Methods of wood cutting, using tools heated by electric current have been long known in the art. Among the known methods are methods of cutting the wood by a hot wire or band reciprocating between the wood-cutting line (for this reference may be made to SU, A, 1314, 142013, 142408, 827293, 885010), by a knife with an electrically heated cutting edge (reference may be made to SU, A, 747720), or by chain and circular saws with electrically heated teeth (see SU, A, 54352, 880731).
The wood heated to 240.degree. . . . 270.degree. C. is known to be destroyed, i.e. it is subjected to a thermal destruction process. This circumstance is used in the known methods to increase the efficiency of wood cutting and to make it completely sawdustfree.
It is apparent to those skilled in the art that such methods are most effective, when the heated cutting part of the tool only produces a thermal destruction of the wood in the tool-feeding direction, avoiding any mechanical contact between the tool and the unheated wood, that is, with an active and steady process of thermal breakdown of the wood as the tool moves forward.
Should there be contact of the tool with the wood mechanical friction of the tool against the wood layers causes the power consumption of the cutting process to be increased and contributes to a substantially earlier wear of the tool, apart from charring the wood layers along the surface of the cut.
The instability of the process of thermal breakdown of the wood and the consequent mechanical contact between the tool and the wood is a major disadvantage of the methods corresponding to the present state of the art. In particular, it has been found by the inventors that the stability of the thermal breakdown process is distributed by the rapidly changing power consumption of the cutting process, which is by no means compensated in the above known methods.
Wood is known to have a nonuniform structure, i.e. a varying density of annular rings, the presence of knots, rots etc. The areas of increased density exhibit a greater heat absorption, and more energy is released, in these areas, by the heated cutting part of the tool, leading to a more intense cooling of the cutting part. In this case, the wood of the increased-density areas is heated to a smaller extent than it would be required for a stable and active process of thermal breakdown of the wood; in other words, in these areas, there is no thermal breakdown of the wood in the tool-feeding direction. As a result, the tool mechanically contacts the wood layers, thus slowing down the cutting process. Due to a mechanical contact and the consequent friction between the cutting part of the tool and the wood, the wear of the cutting part is enhanced, resulting in an early failure of the tool, as well as an increased power consumed for cutting wood.
In the smaller-density areas, heat absorption is low, and less energy is released by the heated cutting part of the tool in these areas, leading to an overheated tool, which also may cause its premature failure.
In addition, the slowing-down of the cutting process in the higher-density areas results in a longer heating of the more porous areas of the wood adjoining thereto along the cutting line, which will cause it to be charred. Carbon, which is an extremely refractory material exhibiting good heat-insulating properties, prevents the heating and thermal breakdown of the wood layers lying beyond the charred layer. Moreover, carbon is strong enough and exhibits abrasive properties. Efforts to overcome the charred layer causes increased wear of the tool and thus, further reduces its service life. Additional power is required to get through the charred layer, thus impairing the effectiveness of the cutting process. The charred surface of the cut results in degraded consumer quality of the wood, and therefore, an additional treatment of the surface proves to be necessary in a number of cases.
The above will be illustrated by a more detailed discussion of known methods of wood cutting and tools for implementing the same.
According to a method disclosed in SU, A, 827293, wood is cut by a wire heated by electric current and reciprocating between two current-supplying roller contacts adjoining the wood on the opposite sides thereof. The device is provided with spring-loaded templates rigidly attached to the current-supplying roller contacts, with an electric current passed through the wire. As the wood is cut, the templates closely contact the wood. Depending on the length of the wire section buried into the wood, the voltage applied to the current-supplying contacts is varied, thereby providing an average heating of the cutting part of the wire introduced into the wood up to a temperature level specified by the cutting conditions (above 400.degree. C.). The maximum value of said temperature of the wire is limited by its strength characteristics.
In this method, as well as in the other known methods, however, the rapidly changing power consumption of the cutting process is in no way compensated. As a result, in more porous areas of the wood, the wire is overheated leading to its more rapid wear. In the denser areas of the wood, the wire is overcooled, with the consequent mechanical friction thereof against the wood and a more rapid mechanical wear. In this case, the looser, more porous, areas of the wood, adjacent the denser areas along the cutting line, get charred. The mechanical penetration of the wire through the denser areas of the wood and the charred layers, is made difficult because of the low specific strength of the wire.
Known in the art are tools having a high specific strength and comprising a carrying part and a cutting part heated by electric current. Such tools include, for example, chain and circular saws disclosed in SU, A, 54632 and SU, A, 880731, and a knife as disclosed in SU, A, 747720, wherein, in order to minimize the power consumption, the cutting part is divided into sections along its length, each section being separately heated. In this case, during the cutting process, the electric power is only consumed at those sections which directly participate in the wood cutting process.
The above tools do not provide stability of thermal breakdown of the wood because of the heat release failing to follow the rapidly changing power consumption of the cutting process. Another factor disturbing the thermal breakdown process stability and producing a mechanical contact of the tool with the unheated layers of the wood is the shape of the cutting part of the tool. In all the known tools (with the exception of the wire), the cutting part is formed by a sharpened edge. Because of a low surface area of the thermal contact between the edge and the wood, and due to a high unit pressure at the edge, the underlying layers of the wood do not have enough time, as the wood is cut, to be heated to a temperature level sufficient for the wood to be thermally destroyed. The tool is thus introduced into the wood largely as a result of mechanical destruction of the wood by the cutting part of the tool, thus substantially increasing the wear of the tool.
Also, because of the varying cross-section of the pointed cutting edge, it is rather difficult to maintain a uniform temperature in the process of cutting, which again disturbs the stability of thermal breakdown of the wood.
In addition, if the cutting part of the tool is formed by a narrow pointed edge heated by electric current, as in SU, A, 747720, and the surfaces of the carrying part project beyond the heated surfaces of the cutting part, the cold side surfaces of the carrying part slow down the penetration of the tool, thus increasing the power consumption needed for wood cutting.
If, on the other hand, the cutting part heated by electric current is made more elongated, in the tool-feeding direction, forming, say, a band (SU, A, 142013) or a tooth (SU, A, 54632), the thermal action on the wood layers adjoining the heated side surfaces of the cutting part is extended, and these layers are charred thus impairing the consumer quality of the cut.